1. Field of the Invention
The present invention relates to an image forming device for forming an image on a printing medium conveyed using a roller, and to a control method for the image forming device.
2. Description of the Related Art
A technique for an image forming device of electrophotography has been known in which a static charge formed on a photosensitive drum is exposed to a laser beam to form an electrostatic latent image based on image data; a toner image is formed by developing the electrostatic latent image with a developing agent; and this toner image is fixed on a printing medium using a fixing roller, thereby performing image formation on the printing medium. The printing medium is conveyed by being absorbed on a conveying belt through electrostatic absorption, for example, and reaches the fixing roller while toner images of four colors are overlapped on each other via photosensitive drums of C, M, Y, and K colors.
The type, in which the toner image is directly formed on the printing medium from the photosensitive drum, is called a direct transfer type. Meanwhile, the type, in which the toner image is formed on an intermediate transfer belt from the photosensitive drum and the toner image formed on the intermediate transfer belt is secondarily transferred to the printing medium, is called an intermediate transfer type.
The photosensitive drum is generally made of metal, and its allowance or thermal change in diameter is relatively small; therefore, variation in linear velocity is also small. Meanwhile, the fixing roller is generally formed using an elastic rubber material because the amount of fixing is large particularly in the case of overlapping the four colors as above. Accordingly, the allowance or the thermal change in diameter of the fixing roller is larger than that of the photosensitive drum; thus, the variation in linear velocity is also larger than that of the photosensitive drum.
Here, when linear velocity difference is caused between the photosensitive drum and the fixing roller, for example, in the aforementioned direct transfer type, the conveyance of the printing medium is affected by this linear velocity difference. For example, when the printing medium is pulled by the fixing roller to make the linear velocity difference between the photosensitive drum and the fixing roller, the conveying velocity changes depending on this linear velocity difference. Therefore, the velocity of the printing medium and the linear velocity of the photosensitive drum (that is, the linear velocity of an image forming unit) does not match with each other, thus leading to deviation of an image formed on the printing medium from an original image in a conveying direction (sub-scanning direction). This deviation is caused by the change of the interval of main scanning in accordance with the ratio between the linear velocity of the fixing roller and the linear velocity of the photosensitive drum, and is called sub-scanning magnification deviation below.
To suppress such magnification deviation in the sub-scanning direction, Japanese Patent Application Laid-open No. 2009-067561 has disclosed a configuration in which a helical gear is used for a roller driving system. Moreover, Japanese Patent Application Laid-open No. 2011-081270 has disclosed a technique for detecting the circumferential velocity of a fixing roller or an intermediate belt and correcting each linear velocity by controlling a driving motor of the fixing roller or the intermediate belt based on the detection result.
However, the method disclosed in Japanese Patent Application Laid-open No. 2009-067561 has had a problem in that the number of gears is increased because of the use of the helical gear, which makes the configuration of the roller driving system complicated. Further, the increase in number of gears increases the torque and the consumption power.
Moreover, the method disclosed in Japanese Patent Application Laid-open No. 2011-081270 has had a problem in that it takes time to match the linear velocities of the respective units; and in the case of the intermediate transfer type, it has been difficult to deal with quick control, for example, for correcting the sub-scanning magnification deviation caused when a printing medium passes through a secondary transfer unit and the fixing roller. This corresponds to, in the case of the direct transfer type, the state in which the printing medium passing through the fixing roller is subjected to the transfer also simultaneously on a rear end side, and is a problem also occurring in the direct transfer type.
There is a need to correct, with a simpler configuration, the magnification deviation in the sub-scanning direction in the case where the conveying velocity of the printing medium changes relative to the linear velocity of the image forming unit.